1. Field of the Invention
The present invention relates to an electrophotographic type image forming apparatus such as a laser printer, a plain paper copier, and a plain paper facsimile, particularly to a mechanism in which silicone gas generated in a fixing unit does not affect a charger.
2. Description of the Related Art
FIG. 10 shows an entire constitution of a conventional laser printer. As shown in FIG. 10, a printer body 1 comprises an optical unit 2 provided with a semiconductor laser device 3 and a mirror 4, a process unit 5 into which a photosensitive body 6, a charger 7 and a cleaning device 8 are integrated, a developing unit 9 into which a toner box 10 is integrated, a fixing unit 11, a transfer unit 12, a sheet feeding cassette 13, a sheet storage portion 14, a sheet hopper 15, a pick-up roller 16, a sheet conveying path 17, a manually sheet-feeding portion 18, a sheet carrying roller 19, a registration roller 20, a sheet conveying roller 21, a sheet discharging roller 22, and a face-down tray 23.
A discharge electrode 25 (charger wire or acicular electrode) is arranged in a case body 24 of the charger 7 as shown in FIG. 11, to provide electrostatic charges on a surface of the photosensitive body 6 by corona discharge to electrify the surface. At that time, a corona wind (air flow) rises in the charger 7. More specifically, a negative electron moves from the discharge electrode 25 (charger wire) o an opposed electrode (the case body) by corona discharge, and at that time, an air flow, so-called corona wind is caused on the process that the air between the electrodes is ionized.
The fixing unit 11 is provided with an upper heat roller 27 and a lower pressure roller 28 made of silicone rubber which are rotatably supported by the main body 26 of the fixing unit 11.
Further, the upper heat roller 27 is in contact with a cleaning roller (not shown) impregnated with silicone oil in order to prevent the upper heat roller 27 from being contaminated by toner. The surface temperature of the respective rollers 27, 28 is maintained high by heater lamps arranged for the respective rollers 27, 28 in order to fix the toner on a sheet. In view of the separability (degree of separability of the molten toner from the surface of the heat roller) of the actually available toner, a separating agent such as silicone oil is necessary for separating the residue of the molten toner from the heat roller. If the separating agent is not used, a problem arises that the residue of the molten toner on the heat roller contaminates the top of a sheet on the following rotation of the heat roller, and additionally contaminates the surface of the opposed pressure roller, resulting in the contamination of the back of the sheet.
A silicone gas with a low molecular weight is generated in a conventional laser printer, because the silicone rubber of the pressure roller 28 and the silicone oil in the fixing unit 11 are vaporized due to the high temperature. The silicone gas is moved toward a direction of arrow B shown in FIG. 11 by a fan (not shown) in the printer body 1, so that the charger 7 is wrapped round by the silicone gas. Moreover, since the printer body 1 is not provided with a dedicated air passage, air flow A is generated which is introduced into the charger 7 and discharged therefrom due to a corona wind on discharging and a light wind by the rotation of the photosensitive body 6, and the silicone gas is involved into the air flow with the peripheral air.
Accordingly, positive ion molecules of a low molecular siloxane contained in the silicone gas are drawn to the discharge electrode (charger wire 25) to which a high voltage is applied, and the low molecular siloxane contained in the silicone gas is deposited on the tip portion etc. of the electrodes, which causes abnormal discharge. As a result, the photosensitive body 6 is ununiformly electrified, which has a bad influence on the image quality.
The constitution that the generated silicone gas is directly exhausted to the outside or that the generated silicone gas is isolated so as to be prevented from entering into the side of the charger 7 is difficult to be realized because the complexity of the internal structure thereof. Besides, the necessity of providing with a fan causes an increase in cost.
Further, it is necessary to clean the discharge electrode 25 early, because the abnormal discharge occurs in the relatively short period of service (10,000 to 15,000 sheets copied).